Fix C++20/gcc-12 issues (Part 2) (#3446)

* Add C++20 3-way comparison operator and fix broken comparisons

Fixes #3207.
Fixes #3409.

* Fix iterators to meet (more) std::ranges requirements

Fixes #3130.
Related discussion: #3408

* Add note about CMake standard version selection to unit tests

Document how CMake chooses which C++ standard version to use when
building tests.

* Update documentation

* CI: add legacy discarded value comparison

* Fix internal linkage errors when building a module

docs/mkdocs/docs/api/macros/index.md

@@ -17,6 +17,8 @@ header. See also the [macro overview page](../../features/macros.md).
 
 - [**JSON_HAS_CPP_11**<br>**JSON_HAS_CPP_14**<br>**JSON_HAS_CPP_17**<br>**JSON_HAS_CPP_20**](json_has_cpp_11.md) - set supported C++ standard
 - [**JSON_HAS_FILESYSTEM**<br>**JSON_HAS_EXPERIMENTAL_FILESYSTEM**](json_has_filesystem.md) - control `std::filesystem` support
+- [**JSON_HAS_RANGES**](json_has_ranges.md) - control `std::ranges` support
+- [**JSON_HAS_THREE_WAY_COMPARISON**](json_has_three_way_comparison.md) - control 3-way comparison support
 - [**JSON_NO_IO**](json_no_io.md) - switch off functions relying on certain C++ I/O headers
 - [**JSON_SKIP_UNSUPPORTED_COMPILER_CHECK**](json_skip_unsupported_compiler_check.md) - do not warn about unsupported compilers
 
@@ -29,6 +31,12 @@ header. See also the [macro overview page](../../features/macros.md).
 
 - [**JSON_USE_IMPLICIT_CONVERSIONS**](json_use_implicit_conversions.md) - control implicit conversions
 
+<!-- comment-->
+## Comparison behavior
+
+- [**JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON**](json_use_legacy_discarded_value_comparison.md) -
+  control comparison of discarded values
+
 ## Serialization/deserialization macros
 
 - [**NLOHMANN_DEFINE_TYPE_INTRUSIVE(type, member...)**<br>**NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(type, member...)**](nlohmann_define_type_intrusive.md) - serialization/deserialization of types _with_ access to private variables